European Patent Specification 0 059 064 B1, Webster, describes a circuit arrangement permitting dimming of a fluorescent lamp. The circuit includes an externally controlled inverter which supplies a low-pressure discharge lamp, typically a fluorescent lamp, over a series resonant circuit. The electrode filaments of the lamp are preheated. During the electrode preheating phase, that is, before ignition of the discharge lamp, the inverter supplies the lamp with current having a frequency which is substantially above the resonant frequency of the series resonant circuit. The switching frequency of the inverter is changed in the direction of the resonant frequency of the series resonant circuit in order to fire the lamp. The required ignition voltage is obtained by the resonant peak occurring in the now effectively tuned resonant circuit. The lamp is operated at a frequency which is somewhat above the resonant frequency of the series resonant circuit, which is now adapted to the lamp.
The lamp can be dimmed or, in other words, the brightness of the lamp can be controlled, by changing the switching frequency of the inverter, and hence the frequency of the lamp current in dependence on the setting of a dimmer. Dimming is obtained by increasing this frequency. The resonance capacity is connected in parallel to the fluorescent lamp. Upon increase of the frequency, the impedance of the capacity decreases, which reduces the lamp current. Thus, dimming of the fluorescent lamp is obtained by changing the frequency of the inverter.
German Patent 33 38 464 (assigned Plankenhorn Kapitalverwaltungs-KG) describes a circuit with a freely oscillating inverter which operates a dimmable fluorescent lamp. Control of the brightness of the fluorescent lamp is obtained by changing the duty cycle of the high-frequency alternating current energy derived from the inverter, in dependence on a setting of the dimmer control.
German Utility Model G 89 15 386 (assigned Zumtobel AG) describes a circuit arrangement permitting dimming of a fluorescent lamp. The dimming of the lamp is obtained by controlling a combination of frequency and duty cycle of alternating current supply delivered to the fluorescent lamp.
All the circuits so far described require comparatively complex circuitry, and have the disadvantage that the fluorescent lamp, immediately after ignition, operates initially at full power, independently of the setting of the dimmer, and before the dimmer control unit can control the frequency and/or the duty cycle of the inverter supplying the lamp in accordance with the setting of the dimmer control.
German Utility Model G 91 00 552 (assigned Trilux-Lenze) describes a different arrangement; the circuit has a half-wave inverter which supplies a fluorescent lamp over a series resonant circuit. Dimming or brightness control of the lamp is effected generally similar to a phase control of a load. A bridging or shunt switch is connected in parallel to the lamp and bridges or shunts the fluorescent lamp in accordance with a controllable phase angle of the lamp current. The control of the phase angle depends on the setting of the dimmer. The current flowing across the discharge path of the fluorescent lamp is thus weakened in accordance with the setting of the dimmer control. Matching the bridging circuit and switching element included in the shunt circuit to the switching phases of the inverter is complex and requires substantial circuitry.